1. Field of the Invention
This invention relates to an overvoltage protection circuit, and more particularly to an overvoltage protection circuit using thyristors with an overvoltage protection function, an overvoltage protection equipment constructed with the same and a power converter also constructed with the same.
2. Description of the Related Art
FIG. 9 is a layout diagram showing an example of a conventional overvoltage protection equipment using thyristors equipped with an overvoltage protection function (hereinbelow called VBO-free thyristors).
In this FIG., 1 is an induction montor, 2 is a power converter that supplies low frequency AC power to the secondary circuit of induction motor 1, and 3 is an overvoltage protection equipment wherein three overvoltage protection circuits 4 constituted by connecting in series a plurality of antiparallel connection circuits consisting of VBO-free thyristors 11 are connected between each phase of power converter 2. When overvoltage is generated in the secondary circuit of induction motor 1 by shut-down etc of the primary side of induction motor 1, the secondary circuit of induction motor 1 is short-circuited to protect induction motor 1 from overvoltage.
FIG. 10 is a specific layout diagram of the conventional overvoltage protection circuit 4 shown in FIG. 9. In this layout, a plurality of antiparallel connection circuits consisting of VBO-free thyristors 11 connected in antiparallel are connected in series; snubber circuits consisting of series circuits of snubber capacitor 12 and snubber resistor 13 are connected in parallel with the respective antiparallel connection circuits; and voltage grading resistors 14 are respectively connected therewith in parallel.
A VBO-free thyristor 11 is a device which, when a predetermined prescribed voltage is applied, can perform turn-on even without a gate signal being applied. Snubber capacitors 12, snubber resistors 13 and voltage grading resistors 14 are provided with the purpose of ensuring uniform voltage sharing between the VBO-free thyristors 11 which are connected in series; their values are the same for each circuit. If we represent the voltage at which firing of VBO-free thyristor 11 takes place without application of a gate signal by VVBO, the turn-on voltage in the entire series circuit constituted by n VBO-free thyristors 11 is n.VVBO.
In the overvoltage protection equipment 3 shown in FIG. 9, when the overvoltage protection equipment 3 is constructed with three overvoltage protection circuits 4 constituted by connecting in series n antiparallel circuits obtained by antiparallel connection of VBO-free thyristors 11, when the voltage applied to both ends of the overvoltage protection circuit 4 reaches n.VVBO, it is turned ON and electrical equipment such as inductor motor 1 and power converter 2 connected to the overvoltage protection equipment 3 is thereby protected from overvoltage.
However, generally it must be ensured that the overvoltage protection equipment is not turned ON for the voltage that is regularly generated by the electrical equipment, which is connected in parallel with the overvoltage protection equipment and is to be protected from overvoltage. Since this regularly generated voltage also includes, for example, surge voltage generated by power converter shown in FIG. 9, a value higher than the rated voltage of the electrical equipment that is to be protected from overvoltage must be selected for the operating voltage of the overvoltage protection equipment 3. In other words, a number of VBO-free thyristor 11 antiparallel circuits to be connected in series must be selected such that the voltage at which the overvoltage protection equipment 3 turns ON is higher than the regularly generated voltage. For example, if the regularly generated voltage is more than n.VVBO and less than (n +0.5).VVBO, the number of VBO-free thyristor 11 antiparallel circuits connected in series must be (n +1). In this case therefore the overvoltage protection equipment 3 does not turn ON unless the voltage applied to the electrical equipment or the voltage generated in the electrical equipment to be protected from overvoltage reaches (n +1).VVBO. Thus, the electrical equipment that is to be protected from overvoltage must be capable of withstanding this (n +1).VVBO voltage. In the case where the electrical equipment is a power converter, it must have a higher level of insulation and more number of series-connected semiconductor devices and other circuit elements than the level and the number determined by the normal voltage.
For example, if VVBO is 6 kV and the regularly generated voltage is 13 kV, the number of series-connected VBO-free thyristor 11 antiparallel circuits to be connected in series is 3, giving an overvoltage protection equipment turn-on voltage of 18 kV.
Consequently, although the ideal protection voltage of the electrical equipment that is to be protected from overvoltage can be set about 14 kV to 15 kV, the turn-on voltage of the overvoltage protection equipment is decided to be 18 kV, so the electrical equipment itself that is to be protected from overvoltage must be such in terms of insulation and construction as to be capable of withstanding 18 kV.
The turn-on voltage of the conventional overvoltage protection equipment changes in stepwise manner. If therefore harmonization could not be achieved between the operating voltage of the overvoltage protection equipment and the voltage generated by the electrical equipment to be protected from overvoltage or the voltage applied to the electrical equipment, size and cost of the electrical equipment were increased due to the resulting requirement for further insulation.
The overvoltage protection circuit 4 shown in FIG. 10 has the function of providing protection for both polarities of the voltage generated by the electrical equipment to be protected from overvoltage or voltage applied to the electrical equipment. However, in cases where the voltage generated by the electrical equipment to be protected from overvoltage or voltage applied to the electrical equipment is a polarity in only one direction, instead of the VBO-free thyristors 11 needing to be antiparallel-connected, series connection would suffice. In such cases also, as described above, if harmonization cannot be obtained between the voltage generated in the electrical equipment to be protected from overvoltage or voltage applied to the electrical equipment and the operating voltage of the overvoltage protection equipment, the electrical equipment to be protected from overvoltage has to be unduly large or expensive due to the requirement for increased insulation.